This invention relates to a reconfigurable antenna array system, and includes an array of dipole antenna elements disposed on a multiple band high impedance surface. The antenna array is configured by changing the resonant frequency of the individual dipoles that constitute the array. At a given frequency band, small changes in the dipoles resonant frequencies allow for the antenna array to be configured so that the reflected radiation forms a beam in the far-field, and can be pointed to selected directions. Larger changes in the dipoles resonant frequencies allow for shifting from one operating frequency band to a different band. This invention has particular applications in satellite radar and airborne communication node (ACN) systems where a wide bandwidth is important and the aperture must be continually reconfigured for various functions. Additionally, this invention has applications in the field of terrestrial high frequency wireless systems.
The prior art includes U.S. Pat. No. 4,905,014 to Daniel G. Gonzalez, Gerald E. Pollen, and Joel F. Walker, xe2x80x9cMicrowave passing structure for electromagnetically emulating reflective surfaces and focusing elements of selected geometryxe2x80x9d. This patent describes placing antenna elements above a planar metallic reflector for phasing a reflected wave into a desired beam shape and location. It is a flat array that emulates differently shaped reflective surfaces (such as a dish antenna). However it does not disclose a system that is reconfigurable and can operate at multiple frequency bands.
The prior art includes U.S. Pat. No. 5,541,614 to Juan F. Lam, Gregory L. Tangonan, and Richard L. Abrams, xe2x80x9cSmart antenna system using microelectromechanically tunable dipole antennas and photonic bandgap materialsxe2x80x9d. This patent shows how to use RF MEMS (Micro Electro-Mechanical Switches) and bandgap photonic surfaces for reconfigurable dipoles. Although this invention lists a number of reconfigurable dipole antenna architectures, it does not disclose the dipole reflector antenna, and it does not show how to use multiple band, high impedance surfaces (a sub-class of photonic bandgap material). Furthermore, in the present invention the dipole array is fed from free space rather than a transmission line.
The present invention also relates to U.S. patent application Ser. No. 09/537,923 entitled xe2x80x9cA tunable impedance surfacexe2x80x9d filed on Mar. 29, 2000 and to U.S. patent application Ser. No. 09/537,922 entitled xe2x80x9cAn electronically tunable reflectorxe2x80x9d filed on Mar. 29, 2000, and to U.S. patent application Ser. No. 09/537,921 entitled xe2x80x9cAn end-fire antenna or array on surface with tunable impedancexe2x80x9d filed on Mar. 29, 2000, the disclosures of which are hereby incorporated herein by this reference. The present invention improves upon the high impedance surface of U.S. patent application Ser. No. 09/537,923 entitled xe2x80x9cA tunable impedance surfacexe2x80x9d, and provides a method of broadening the surface operating bandwidth.
As an aid in understanding the principle of operation of this invention, the prior art is instructive. Turning to FIG. 1a, a dipole element 1, located xcex/4 away from a metallic ground plane 2, is shown. An incident plane wave 3 is reflected from the ground plane 2 and also scattered from the dipole element 1. When the dipole element is at its resonant length, (i.e., its length 1d is appoximately equal to half of the effective signal wavelength, 1d≈xc2xdxcexeff), scattering from the dipole is very strong and the effect from the ground plane is negligible. Thus, the total field has a reflection phase of approximately 180xc2x0 (at the plane of the dipole). If the dipole is far from its resonant length, then scattering from the dipole is weak and the reflection phase, due primarily to the ground plane, is approximately 0xc2x0 (at the plane of the dipole). Therefore, the phase of the reflected field from the dipole element can be adjusted by making small changes in the length of the dipole.
As an example, simulation that shows the behavior of the reflected phase versus dipole length is represented in FIG. 2. The simulation assumes that the dipole element is part of an infinite array, and is located in free space, xcex/4 away from the ground plane. It further assumes a operating frequency of 11.8 GHz and that the dipole strip is 0.1 inch (CGS) in width. The dipole length varies from 0.1 to 0.8 inch. As can be seen in FIG. 2, the reflection phase of the dipole element can be tuned over a wide range, about 85xc2x0, for a length change of only 0.05 inch
FIG. 2a demonstrates a technique of varying the length of a dipole element using RF MEMS technology. The dipole element 20 is segmented into a main segment 22 and a plurality of smaller segments 21. Each segment is interconnected to the adjacent one by an RF MEMS switch 23. By opening or closing the RF MEMS switches 23, the dipole length can be changed in steps equal to small segment length plus switch length. In this example, the small segments are approximately 200 xcexcm in length, and the switches are about 100 xcexcm long. Consequently, when a switch is opened, the dipole length is increased by 300 xcexcm. This corresponds to approximately a 10xc2x0 change in the reflected phase. By making the segments and/or switches smaller, a finer phase tunability can be achieved.
These length-changeable dipole elements can be incorporated into an array, disposed above a ground plane, and tuned to create a reflection phase gradient across the array. In this configuration, the total reflected wave forms a beam, which can be steered to incremental angular directions, by creating uniform phase gratings across the array. FIGS. 3a and 3b illustrate this concept for a linear array and a planar array respectively. This type of array can then serve as a stand-alone antenna or as a subreflector to another primary reflecting surface, such as a Cassegrain antenna.
However, the approach described in the immediately preceding paragraph has bandwidth limitations, as this will now be explained. Each dipole element of the array is modeled as a series resonance circuit 40, located xcex/4 from a short circuit 41, as illustrated by FIG. 4. An infinite array approximation is assumed. The values of the inductance and capacitance are functions of the dipole length, width, and unit cell size. When the short circuit is located xcex/4 from this susceptance (LC circuit), it appears as an open circuit across the susceptance and the reflection coefficient of the element can be tuned such that the reflection phase takes values over a full range of angles as shown in FIG. 2. However, at a frequency where the distance between the dipole and the ground plane is xcex/2, the ground plane effectively shorts out the dipole and the reflected phase is locked at 180xc2x0, regardless of dipole length (no tuning is possible). Thus, as the array operates over a range of frequencies, inducing the distance between the ground plane and the dipole to vary between xcex/4 and xcex/2, the tuning range of the reflected phase becomes more and more limited. The present invention overcomes this limitation by placing the dipole array over a high impedance surface.
A high impedance surface is a filter structure which has the capability of reflecting an incident plane wave with a 0xc2x0 phase shift. The basic structure of a high impedance surface is shown in FIG. 5a, and can be fabricated using multi-layer printed circuit board technology. Preferably hexagonal or square metal patches 50 are disposed on the top surface and connected to a lower metal sheet 51, by plated metal posts 52. The high impedance surface 54 acts as a filter to prevent the propagation of electric currents along the surface, over the frequency stopband. Therefore, unlike conventional conductors, propagating surface waves are not supported within the frequency stopband. Furthermore, incident plane waves are reflected without the phase reversal that occurs on an ordinary metal surface. FIG. 5b shows the reflection phase of the high impedance surface 54. The bandwidth over which the reflected phase lies between xe2x88x9290xc2x0 and 90xc2x0, is given by:       Equation    ⁢          xe2x80x83        ⁢    1    ⁢          :        ⁢          xe2x80x83        ⁢                  Δ        ⁢                  xe2x80x83                ⁢        f                    f        o              =                    L        C                            μ        ϵ            
where L and C are related to the equivalent circuit model (see FIG. 5c) of the high impedance surface (and not to be confused with the dipole model of FIG. 4). As shown in FIG. 5c, the capacitance C is due to the proximity of the top metal patches 50, and the inductance L originates from the current loops within the structure. f0 is the frequency for which the reflected wave has a 0xc2x0 phase shift, xcexc and xcex5 are the material permeability and permettivity respectively.
In accordance with this invention, an array of reconfigurable dipole antennas is disposed above a high impedance surface. In this manner, the dipole elements do not have to be placed xcex/4 away from the ground plane as required by the prior art. This has the effect of making the system geometry independent of the frequency of operation. Thus, the operating frequency can be changed without having to alter the relative geometry of the array and the back plane, for the purpose of maintaining a xcex/4 distance between them. This allows the array to maintain tunability over the full bandwidth of the high impedance surface.
The present invention provides an apparatus and method for tuning the array by changing the length of the dipole elements using RF MEMS technology, which overcomes the problems posed in the prior art, by the use of photoconductive switches.
This invention provides a multiple band, reconfigurable electromagnetic reflecting antenna system which can be reconfigured to operate at multiple frequency bands; the user can select the operating frequency band from a range that can be anywhere within the total surface bandwidth. Furthermore, at a given operating frequency band, the antenna system is capable of forming an antenna beam in the far-field and pointing the beam to selected directions.
In accordance with this invention, an array of dipole antenna elements is fabricated on top of a multiple band, high impedance surface. Reconfigurability is achieved by varying the resonant frequency of each dipole which is a function of dipole length. Thus by changing the dipole length, one can vary the resonant frequency of the dipole. Each dipole antenna element is segmented, and the segments are interconnected with RF MEMS (Micro Electro-Mechanical Switches) which can be opened or closed to change the length of the dipole. Small changes in dipole length allow for beam steering and forming a beam in the far-field, while larger changes allow for changes in the antenna array operating frequency band.
This invention further provides a method of increasing the bandwidth of the high impedance surface that supports the array of dipoles, by increasing the surface inductance.